A cataclysmic flood split Britain from France hundreds of thousand years ago, in a violent act of nature that carved out the white cliffs of Dover and set the course of history for a new island.

High-resolution sonar images of the English channel collected over more than 20 years have revealed a deep scar in the limestone bedrock caused by a torrent of water 400,000 years ago. At the time glaciers reached from the North Pole to a point north of the Thames, and England's southern coast could be reached from France by a broad land bridge.

The images show a sub-marine channel that is now about 90metres deep with scour marks and land forms shaped by the overwhelming rush of water.

Sanjeev Gupta and Jenny Collier, researchers at Imperial College London, who compiled and analysed the images, believe a vast freshwater lake formed over thousands of years in what is now the southern part of the North Sea. Fed by the Rhine, the Thames and other European rivers, the lake spread about 400 miles from East Anglia across the Netherlands to Germany and about 200 miles from the north coast of Norfolk to the strait of Dover. It was hemmed in by glaciers to the north and a large chalk dam at the strait.

The images suggest the chalk barrier at Dover was breached by the rising lake, releasing water that gouged the land beyond, creating a giant channel between the two land masses. "This would have been a torrent of water carving out a huge valley through this wild landscape," said Dr Gupta. "There would be powerful eddies, with huge boulders and chunks of chalk ... thrown around in the surge."

The team estimates the surge released between 200,000 and 1m cubic metres of water a second, equivalent to 100 times the discharge of the Mississippi river.

"Prior to this ridge being breached, Britain would have been a promontory with a very clear connection to France, but once this happened every time there were high sea levels Britain became an island," Dr Gupta said. "Britain's identity and historical development was shaped by what was essentially a chance geological event."

The researchers, whose findings are reported in the journal Nature, believe a second massive flood completed the job of separating Britain from France around 180,000 years ago. Another vast glacial lake formed from rivers off the east coast of Britain and was temporarily contained to the south by an earth ridge spanning from north of the location of present-day Ipswich across to a point which is now The Hague. Water released into the Dover strait widened the channel to more than nine miles in some regions.

Philip Gibbard, a geologist at Cambridge University, said the megafloods that created Britain were on a par with the largest known on Earth, including an event that created Lake Missoula in Washington state at the end of the last ice age.

The second flood followed less severe glaciation and may have been witnessed by early humans, the researchers believe.

"This prehistoric event rewrites the history of how the UK became an island and may explain why early human occupation of Britain came to an abrupt halt for almost 120,000 years," said Dr Gupta.